1. Field of the Invention
The present invention relates to an improvement to a pipette for sampling and dispensing adjustable volumes of liquids.
2. Background Information
The invention relates more particularly to an improvement to a pipette of the type comprising a pipette body extending along a longitudinal axis and a piston mounted in sliding fashion along this axis inside the said pipette body so as to define a chamber of variable volume emerging at a sampling orifice, said piston being driven over a limited distance of travel in the direction of the sampling orifice by means of pressure manually exerted on a push rod mounted in sliding fashion in the pipette body, and in the opposite direction by the reaction of elastic return means until it abuts against an axial extremity of a metal sheath extending along the said longitudinal axis inside the pipette body. The sheath has an external thread which works by engaging with a thread inside the pipette body in such a way as to move the sheath longitudinally when driven in rotation, in order to adjust the distance of travel of the piston in the opposite direction and the volume of the chamber. Moreover, the sheath is capable of being driven in rotation by a fine adjustment knob integral with the sleeve, and intermeshes with means for displaying the volume to be dispensed. The push rod is mounted in sliding fashion in the sleeve.
In this type of pipette, the volume to be dispensed is usually adjusted with the aid of thumb and forefinger. These two fingers are applied on opposite sides of the fine adjustment knob accessible via windows on opposite sides of the pipette body. While the fine adjustment knob makes it possible to precisely adjust the volume to be dispensed, it can be turned over only a limited angle of rotation on each action of thumb and forefinger, this distance being related to the width of the windows provided on the pipette body. Now, the operator using the pipette will, in certain cases, have to successively dispense widely-differing volumes, and in order to set these values on the pipette, the sheath has to be driven in rotation a large number of times calling for the knob to be operated many times by thumb and forefinger. Beside the drawback of the tedious manner in which the pipette is adjusted to widely-differing values, if operators are wearing gloves, the skin of the gloves often snags and tears in the clearance provided between the knob and the edge of the windows provided on the pipette body.